We previously reported the isolation and characterization of a transducing phage of P. aeruginosa, designated DMS3, originally isolated from a clinical strain of this organism. Recent studies in our lab have uncovered a new role for this lysogenic bacteriophage - the ability to inhibit the formation of surface- attached communities (known as biofilms) under all in vitro conditions tested. DMS3, when lysogenizing the laboratory strain P. aeruginosa PA14 or several clinical strains, results in a lysogen unable to make a biofilm under every condition we have tested, including growth on a variety of carbon sources (glucose, casamino acids, LB, arginine, organic acids) and on a variety of surfaces (i.e., PVC & polystyrene plastic, glass), and does so without affecting bacterial growth, indicating that lysogeny by phage DMS3 interferes with a central pathway required for biofilm formation by P. aeruginosa. While most of our studies have involved phage DMS3, we also have isolated several additional phages from clinical isolates of P. aeruginosa that also inhibit biofilm formation by P. aeruginosa. Preliminary evidence suggests that one such phage, designated DMS577, is distinct from DMS3. Taken together, these data indicate that phage DMS3 and other phages can modulate the ability of P. aeruginosa to initiate a surface-attached lifestyle. [unreadable] [unreadable] We hypothesize that lysogeny by phage DMS3 or DMS577 blocks biofilm formation by interfering with an essential component of the biofilm formation pathway. The interaction of bacteria and their phages are striking examples of polymicrobial infections resulting in the bacterium acquiring new virulence factors, evolving new pathogenic traits and increasing the ability of the phage-lysogenized microbe to cause disease in a wide range of hosts. Our data suggest that as part of a polymicrobial infection with P. aeruginosa, bacteriophages DMS3 and DMS577 may have the ability to influence the nature and outcome of infections, including chronic suppurative otitis media. [unreadable] [unreadable] To test the hypotheses above, we propose the following Specific Aims for this R21 application: Specific Aim I. Determine the phage DMS3-encoded functions required to inhibit biofilm formation. Specific Aim 2. Determine the step in biofilm formation blocked by phage lysogeny. Specific Aim III. Determine DMA sequence of phage DMS577 and the mechanism by which DMS577 interferes with biofilm formation. [unreadable] [unreadable] [unreadable] [unreadable]